Processes of brightening and passivating cadmium and zinc



Patented Sept. 9, 1952 PROCESSES OF BRIGHTENING AND PASSI- VATIN G CADMIUM AND ZINC Francis X. Thomson, Delaware Township, Cam I den County, N. 3., assignor to Radio Corporation of America, a corporation of Delaware 7 No Drawing.

This invention relates to processes of brightening and passivating metallic surfaces. More particularly, the invention relates to processes of treating surfaces of cadmium, zinc, or alloys of these two metals.

In order to prevent rusting of objects made of iron or steel, and at the same time, to enhance their appearance, it is common practice to electroplate the objects with thin coatings of other metals more resistant to atmospheric corrosion. Among the metals commonly used for this purpose are cadmium and zinc. However, it is a characteristic of cadmium and zinc that freshly electrodeposited surfaces of these two metals ,quickly lose much of their luster, due to the formation of thin surface films of oxides, as a result of exposure to air.

In order to restore the brightness of surface coatings of cadmium and zinc, various methods have been used with more or less success. In most cases, however, brightness of the coating gradually decreases with age and, eventually, the original luster completely disappears. One of the methods previously used to brighten the surfaces of electrodeposited cadmium or zinc has been to dipthe plated article ina dilute solution of nitric acid. This has the disadvantage, however, of making the surface more susceptible to atmospheric corrosion than an untreated surface. The use of various strong oxidizing acids as brightening agents is also well known. Although the surfaces thus treated resist ordinary atmospheric corrosion, these agents have various disadvantages in that they are not free rinsing, they are usually expensive, and they dissolve a considerable amount of cadmium or zinc from the plated article.

The process of the present invention has none of the above mentioned disadvantages and, on the other hand, is economical and simple to operate. Furthermore, the process can be used satisfactorily in automatic processing machines.

In general, the process comprises rinsing the plated part, dipping in dilute nitric acid, and then dipping in a solution of an alkali metal dichromate preferably containing an organic Wetting agent. The treated part is then rinsed in Water and dried.

One object of the invention is to provide an improved method of brightening and passivating a surface of substantially pure cadmium or zinc.

Another object of the invention is to provide an improved method of brightening and passivating a surface of electrodeposited cadmium or zinc.

Another object of the invention is to provide Application October 26, 1949, Serial No. 123,798

7 Claims. (Cl. 148-6521) an improved method of rendering a surface of substantially pure cadmium or zinc or alloys of the two metals resistant to the pheric corrosion.

A further object of the invention is to provide an improved method of brightening and passivating a surface of substantially pure cadmium or zinc or of cadmium-zinc alloys without dissolving any appreciable, quantities of the metal.

effects of atmos- Still another object of the invention is to pror vide an improved process of brightening and i passivating a surface of substantially pure cadmium or zinc in which the results are long last,-

ing.

Other objects and advantages will be more apparent and the invention will be better understood from the further description which follows Example A cadmium plated part is, first, thoroughly rinsed in cold runningwateroin order to remove, all traces of the plating bath.. .The rinsed.

part is then dipped in a bath of 0.5% nitric acid for 20-30 seconds. The nitric acid bath.

consists essentially of a dilute, aqueous solution of the acid with the ingredients being present in a ratio of 0.5 cc. concentrated nitric acid in cc. of solution. The article is then re-, moved from this solution and, Without rinsing.

off the solution clinging to the surface, is dipped for 20 30 seconds in a bath consisting essentially of 3 ounces of potassium dichromate and Zounces of a wetting agent per gallon of water. Any other alkali metal dichromate; for example, sodium dichromate, may be used and also any organic wetting agent which can be dissolved in the solution. Examples of suitable wetting agents are the well known alkyl-aryl polyether alcohols such as Triton NE (Rohm and Haas), sodium aryl alkyl sulfonate (nacconol NR, Allied Chemical and Dye Corporation), polyethylene glycol tertiary-dodecylthioether, and dodecyl toluene sodium sulfonate. The article is then removed from the bath, rinsed in running water maintained at about -210 F. and dried in hot air.

Once started, the process should be carried through to completion for best results. That is, the article should not be permitted to stand for any appreciable time between successive steps in the process. The concentrations of ingredients in the treating baths may be varied somewhat although the concentrations given in the example are preferred. In the first bath, the nitric acid concentration may vary between about 0.1% and tained. Without a wetting agent, the dichromate solution film tends to be discontinuous.

This leads to early deterioration. of the treated;

surface as well as non-uniform"characteristics When a wetting agent is included, thedichro-- mate solution wets the metal surface-uniformly.

The cadmium or zinc surface which is treated should be substantially pure metal of either one of the two metals or of any alloy of the two. Substantially pure surfaces are obtained by electrodep'osition, chemical deposition, or vapor deposition in vacuo. Ordinary cast metal is usually:

not sufiici'entl'y pureforgoo'd resultsto be ob: tained and is not intended to beusedflwith: the present process.

Although. a '-30 .-se'cond dip in dilute nitric acid is preferred, as: stated in. the example, dips mersion time is: longer than about 40 seconds,

metal'will begin to dissolve from the surface in I appreciable amounts although. the quality of the resulting product will notbe diminished to any extent. The maximum immersion time is thus not critical except from the standpoint of wasting metal. The 20-30 second immersion in the second bath containing the-dichromate-and the wetting'a'gent is'even less critical although this time range is preferred. Longer times than seconds produce no further improvement and are merely a waste of time.

The final rinsing stepinay take placev incold water; if: desired; Warnrwater is: used to aid rapid. drying. and; rapid drying is preferred to avoidany tendency toward'staining of the surface. 'I'h'isfis als'o'the reason why drying in hot air is-preferred.

There:- has. thus been described 5 an Y improved process. of brightening. and passivating surfaces of substantially :pure cadmium or zinc. or alloysof the two metals... Treatingwith dilute nitric acid alone. is much less satisfactory and I it was also :found: that. a. single bath containing both the. nitric acid and the di'chromate didnot. produce theiximproved :results. cleaningmethodst areknown utilizing baths containing; sulfuric acid? and potassium dichromate,

Although previous 4 this type of bath likewise will not produce the excellent results of the present method on cadmium or zinc surfaces.

I claim as my invention:

1. A process of brightening and passivating a metallic surface of at least one substantially pure metal from the class consisting of cadmium and zinc, comprising treating said surface with an aqueous solution of nitric acid in which the acid concentration is about 0.1 to 3% by volume, then treating ..said: surface with an aqueous solution consisting essentially of about 1 to 6 ounces of analkali dichromate salt and about 1 to 8 ounces of' a. wettingv agentiper gallon of water, rinsing .thetreated surface, and drying, said process being carried out without permitting the article being treated'to stand for any appreciable time between successive steps.

2. A process according to claim 1 in which said surface is an electrodeposited layer of cadmium.

3; A process according to claim 1 inwhich'said surface is anelectrodeposited'layeriof zinc.

4. A process accordingto claim .1 inwhichcthe' nitric .acid concentration is'0.5-% b'yvolume; thealkali metal dichromate is potassiuzmdichromate present in the amount of 3 ouncestperrgallon' and the wetting agent iss'd'odecyl toluene sodium: sulfonate present in" the amount of 2...ounces:1per' gallon.

5. A process according. to clairnl inwhichi the surface is immersed in said nitricacidsolutiorr. for from 20 to 30' seconds and: then'immersed in" the solution containing; saidl dichromateia'nd said;

wetting agent for fromZO-B-O seconds;

6 A process according to claim 1 in whichr'the' final rinsing treatment is. carried-put with'water:

at a temperature of l-210". F;

'7. A. process accordingto claim 61in which the finally rinsed. surface-is dried: in hot air.

FRANCISX; THOMSON;

REFERENCES; CITED Thefollowing references are of record in the file of this patent:

UNITED STATES'JPATEN-TIS Number Name Date:

2,035,380" .Wilhelm Mar;.24,1.1936 2,186,579 Dubpernell' et .al; .J3.l1 9; .1940. 2,446,060 Pray et a1. July'27,l1.948 2,465,247 McBride .Mar; 2.2:, 1949i 2,477,310 McLean et'al; .l. July 26,. 194.131v 2,479,423 Snyder1 .Aug;1.6,.1949

OTHER REFERENCES.

Iron Age, July 26, 1945, pages 61' and 143. 

1. A PROCESS OF BRIGHTENING AND PASSIVATING A METALLIC SURFACE OF AT LEAST ONE SUBSTANTIALLY PURE METAL FROM THE CLASS CONSISTING OF CADMIUM AND ZINC, COMPRISING TREATING SAID SURFACE WITH AN AQUEOUS SOLUTION OF NITRIC ACID IN WHICH THE ACID CONCENTRATION IS ABOUT 0.1 TO 3% BY VOLUME, THEN TREATING SAID SURFACE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF ABOUT 1 TO 6 OUNCES OF AN ALKALI DICHROMATE SALT AND ABOUT 1 TO 8 OUNCES OF A WETTING AGENT PER GALLON OF WATER, RINSING THE TREATED SURFACE, AND DRYING, SAID PROCESS BEING CARRIED OUT WITHOUT PERMITTING THE ARTICLE BEING TREATED TO STAND FOR ANY APPRECIABLE TIME BETWEEN SUCCESSIVE STEPS. 